We use MFS as well for our 401(k) plan manager. MFS will use funds from other sources other than their own. Your company has to ask (I don't know how big your company has to be before MFS will listen).

For several years, our plan carried several MFS funds along with the Vanguard S&P Index fund (VFINX). They also carry a Fidelity and a Putnam fund for us. They recently dropped Vanguard and added Deutsche Equity 500 Index Invm (BTIEX- another S&P 500 Index fund). I'm not sure why the change. The expense ratio is a little higher on BTIEX.

So you should not be limited to just MFS funds (although they probably want you to be).